1. Technical Field
This invention relates to a method and apparatus for polishing a semiconductor wafer, and, more particularly, to an improved method and apparatus so as to obtain a substantially uniform polishing action across the surface of the wafer.
2. Description of the Prior Art
Various methods and tools for polishing a semiconductor wafer are known in the art. In general, these tools include upper and lower plates, between which wafers are positioned for polishing. In operation, the two plates are moved relative to each other, and a slurry, consisting of an abrasive solution with or without an etching reagent, is fed between the plates to grind and flush away the material removed from the wafer.
During polishing, it has been found, however, that the load imposed on the wafer leads to a higher concentration of slurry contacting the wafer edges, than its center. As a result, there is a greater polishing action at the edges, thus causing center-to-edge non-uniformity in thickness and poor flatness of the wafer.
Efforts have been made in the art to obtain a more uniform polishing action across the wafer. For example, U.S. Pat. No. 4,313,284, issued to Walsh on Feb. 1982, discloses a method and apparatus for adjusting the surface shape of the upper plate or wafer carrier. In carrying out the method, a vacuum source is connected to the carrier, so as to apply a pressure difference which distorts the carrier into a concave shape. This shape is sought so that the carrier surface will conform to that of the lower plate or turntable (which mounts a polishing pad), which distorts from thermal and mechanical stress during polishing.
Another approach is disclosed in U.S. Pat. No. 4,450,652, issued to Walsh on May 29, 1984, where a constant temperature is maintained on the top and bottom surfaces of the turntable to maintain the wafer carrier and the turntable at the same thermal bow distortion. The temperature differential is maintained constant by sensing the temperature of the polishing pad, then regulating the pressure applied to the wafer.
In both references, the curvature of the wafer carrier surface is distorted so as to conform to the curvature of the turntable. Also, the distortion is obtained by changing the pressure applied to the wafer carrier. Neither reference seeks, however, to regulate the curvature of the wafer carrier surface, so that it is bowed with respect to the turntable, so as to produce a different degree of polishing action at different points across the wafer surface.